Remove addClass() and removeClass() and replace with classes.add() and classes.remove().

Some drive-by style clean up along the way.
Resolving warnings in control library.
Merge branch 'dom_utils' of https://github.com/justinfagnani/leaflet.dart into justinfagnani-dom_utils

Conflicts:
	lib/control/zoom.dart
Merge branch 'justinfagnani-dom_utils'